The present invention relates to impact drivers, a category of rotary power tools intended for use in high torque driving applications. Pulses of torque are generated in such tools via a hammer and anvil arrangement mounted between the driveshaft and output shaft.
A typical arrangement is shown in US Patent Publication No. 2006/0237205 A1. A driveshaft is coupled to a hammer so that rotation of the driveshaft normally rotates the hammer. The hammer contacts an anvil that is integral with an output shaft. When the output shaft encounters little resistance, the anvil rotates along with the hammer. When high resistance to rotation is encountered, the anvil may slow or halt altogether. However the coupling of the hammer to the driveshaft is such that the hammer will repeatedly draw away from the anvil and then spin forward with increased velocity to strike the anvil and provide a pulse of torque, this impact occurring as many as two times per revolution of the driveshaft.
Because it may damage screws or bits not intended for bursts of high torque, an impact driver is generally considered undesirable for low torque applications, and a typical user may be obliged to carry with him a more conventional drill for these purposes. Since the devices operate so similarly, it may seem especially undesirable that one should have to purchase, maintain, and make use of two distinct tools where one might suffice. As such, multifunction drivers which provide different operational modes have become common. A disadvantage of existing hybrid designs is that they are bulky and/or heavy since the housing must accommodate means for achieving all modes.